1. Field of the Invention
The present invention relates generally to the field of digitally switched multiple subscriber communications systems and more particularly to a distributed control implemented on a microprocessor per line or per erlang or per security block basis, centered around a digital switching matrix. A telephone central or local office implementing the aforedescribed distributed control apparatus and method is described.
2. Description of the Prior Art
In modern telephone switching systems, it is presently required that large amounts of data indicative of the status of the lines and trunks served by such a switching system, together with required actions by the switch in response to various line and trunk status conditions be stored. Representative data is subscriber class of service, trunk class of call, subscriber restrictions, directory number to equipment number translations, equipment number to directory number translations, translation of number code to switch action, i.e. area and office code translations with alternate routes, etc. In a centralized control system of the prior art, this data is available in a common memory, which is duplicated for reliability and accessible by common control computers for serial operations upon the extracted data.
In load sharing, or multiprocessing common control systems of this prior art, more than one processor must access a common memory to obtain data at the same time. Various interference problems exist in such a system which results in an effective loss of throughput, which loss increases as the number of computers increases.
Decentralized control systems incorporating distributed control functions have evolved in the prior art. A prior art switching system wherein stored program controllers are distributed throughout the system is described by U.S. Pat. No. 3,974,343. Another prior art switching system is described by U.S. Pat. No. 3,860,761 wherein a progressively controlled switching system utilizing register control couples an entire call at a time rather than the originating and terminating call halves as described by the present invention.
Prior art systems have concentrated on obtaining a high efficiency for the processing function. Multiprocessing was brought in to provide more processing capability but still with the objective of not providing more capability than was necessary. This contributed to undesirable interactions between software packages wherein the modification or addition of a feature could interfere with the current working of other features in a largely unpredictable manner. This in turn led to a practice of exhaustive testing of software packages whenever a feature or a traffic sensitive quantity was changed (sometimes called regression testing). The larger the software packages became the more testing needed to be done, constantly retesting old features to ensure that they remained operable.
A major reason for this prior art problem lies in the common control system architectures of the prior art wherein a stored program control processing function shares itself in time between a multitude of tasks which occur on demand of the randomly originated and terminated traffic. Such architectures also permit software errors and temporary hardware errors to cause the computer program to "jump" to undesired and unpredictable memory locations, thereby disrupting the correct operations of the total software package.
In accordance with the present invention, this cycle of regression testing is eliminated by providing a system architecture which permits a processing function to be assigned to each call for the duration of that call.
In accordance with one aspect of the present invention, each call in process has its own dedicated computer system, a microprocessor, handling it independently of the other calls which are being simultaneously processed. Thus, a system architecture is achieved wherein a dedicated processor is provided per call. This dedicated processor/per call concept permits software packages to be designed such that they do not have to be interrupted to handle another call. The present invention goes further than assigning a processor function per active call in process, extending the principle to embrace the provision of a processing function dedicated to each termination (line or trunk) at the central office. The processing function may be located remotely from the central office at a remote concentration point or even in the telephone instrument.
In addition, a central communication switching path, not only for the speech signals but also for the overhead traffic, created by the need for the individual processing functions to communicate from time to time with each other and with certain centralized functions such as centrally stored data, maintenance modules, man/machine interfaces, etc. is provided.
When a plethora of processing functions operate independently but interactively with each other in a largely asynchronous manner to one another, this represents a distribution of the control function from a central location to each individual line and/or trunk termination. The communication between processing functions is via a hardware interface. Each processing function is concerned only with the features of the particular line/trunk it is assigned to serve. Thus, it can be tested once exhaustively and thereafter will continue to work with similarly tested functions via the common interface. The addition of a new feature into a processing function associated with a particular line may not function correctly (before it has been exhaustively tested) with another existing line not having the new feature but it cannot prevent two such existing lines from operating with one another because it is not involved in any way with the connection of a call between two existing lines. Sufficient hardware safeguards are present in the communications interface to detect erroneous transmissions between processing packages and bringing them to the attention of maintenance personnel. Also, it may be made impossible for one processing function to alter the operating instructions for any other processing function. In the present invention all that a processing function can do is to provide data via a hardware interface on which the distant processing function may choose to operate in accordance with its own set of stored instructions. One such hardware communication interface is the continuously expandable switching network described in the U.S. patent application of K. Geisken-J. Cotton, Ser. No. 909,583, now U.S. Pat. No. 4,173,713, a continuation of Ser. No. 766,396 now abandoned, filed on Feb. 7, 1977, and assigned to the same assignee as is the present invention.
By utilizing a processor for each termination, functions which in the prior art have required hardware logic and electromechanical and audio devices, can be done under software control by the processor. In the present invention these functions are performed on a per line/trunk basis, including but not limited to, ringing the line, tone detection and generation, testing the line, analog-to-digital conversion, etc. to permit transmission and switching of information in time division multiplex (TDM) formats.
Current time division multiplex (TDM) transmissions, as is well known, transfer analog amplitude information as digital values, such as by delta modulation or by pulse code modulation (PCM) wherein, amplitude information is sampled at periodically consecutive points of time and represented by binary words. Such binary words are transferred as data bytes in periodically consecutive time slots, which time slots, when allocated to a communication link, form a time channel. Switching of time slots between channels by time switches using time slot interchanges is well known and described in detail, for example in U.S. Pat. No. 3,770,895.
Analog-to-digital conversion logic, either in a line circuit interface such as described in the application of, R. Treiber, Ser. No. 903,458, now U.S. Pat. No. 4,161,633 a continuation of Ser. No. 773,713, filed on even data herewith, now abandoned, assigned to the same assignee as the present invention, or in a digital subset is controlled by microprocessor logic, with the same microprocessor logic being also adaptable to control central office switching to central office data bases. Each such subset or group of subsets is controlled by a dedicated microprocessor including a programmable memory including a memory update capability through its digital channel to the central office. While a single microprocessor could be dedicated to service each subscriber set, in accordance with an aspect of the invention, a group of subscribers can be serviced by each microprocessor by means of local distribution multiplexors utilizing microprocessor logic such that a common program memory would service and be accessible by, for example, thirty to sixty subscribers.
In accordance with the present invention, the aforedescribed control functions are distributed to the individual lines and trunks to the degree that it is determinable by the time and space switching functional elements of the central office whether the programmed line and switching control functions are local, at the central office, or distributed further down line. The distributed control technique of the present invention is most advantageously utilized in conjunction with an essentially non-blocking switching network which switches not only voice and data traffic (revenue traffic) but also switches "overhead traffic" accessing the various data bases; such as: translator inputs, man-machine interface data, billing and traffic collection devices, etc. While a number of digital electronic switching systems may be utilized with the present invention, a particularly suitable switching system is described by the aforementioned K. Geisken-J. Cotton, patent application. In accordance with a further aspect of the present invention, the digital switches have coupled thereto switching control instruction over the same paths which couple the speech signals thereto, since the speech path is the only available path over which a remote subscriber subset can send such control data, hence, digitized speech and digital control signals are multiplexed on a common communication path through the group switch to both establish, maintain and terminate communication between calling and called subscribers.
In accordance with another aspect of the present invention a half call technique is described wherein, for example, one line may comprise a two-wire coin box line while another line may comprise a business line to a PABX. Each processing function associated with each such line is programmed to know its own class of service and line signalling interface and also to know the common central interface and how to communicate via the group switch to another half call machine. Thus, a connection may be made between the two lines without having to know the complexities of how to handle all possible combinations of calls. In some instances it is required to transmit through or transmit signals in a forward and backward direction. These signals must be connected to the standard interface so as to be intelligible to other half call units.